Cas system for condyle measurement

ABSTRACT

A method of determining a reference point on condyles of a femur uses a computer assisted surgery system having a digitizer with perpendicular first and second planar surfaces, the digitizer being locatable and trackable in three dimensional space by the computer assisted surgery system. A digitized plane is created using the digitizer, the digitized plane being a posterior plane and/or a distal plane of the condyles. The posterior plane and the distal plane are respectively defined by the first and second planar surfaces of the digitizer. A location of the reference point in the digitized plane is then determined using the computer assisted surgical system. The reference point is a most remote point of the femur within the digitized plane.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority on U.S. Provisional Patentapplication No. US 60/761,767 filed Jan. 25, 2006, the entire contentsof which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to a Computer Assisted Surgery(CAS) system for use in orthopedic knee surgery, and more particularlyto a CAS system, as well as an associated device and method, formeasuring a femoral condyle during such a surgery.

BACKGROUND OF THE INVENTION

The use of Computer Assisted Surgery (CAS) systems for orthopedicoperations in general, and for Total Knee Replacement (TKR) surgery inparticular, is becoming increasingly more commonplace with advancementsin CAS systems and equipment that ensure improved accuracy, fail safeoperation and increasing ease of use.

During TKR surgery, the accuracy of cuts made in the patient's boneprior to the installation of any implant, and particularly the resectioncut made in the condyles at the distal end of the femur, is importantand contributes significantly to the success of the total kneereplacement.

TKR surgery requires several precise cuts to be made in the femur andtibia, including the femoral condyle distal resection cut mentionedabove, such that the implant fits correctly and best replicates thegeometry of a natural healthy knee. Generally, in TKR surgery, thedistal ends of both the femur and tibia are completely resected suchthat these joint articulation surfaces can be replaced with prostheticimplants. To perform these resection cuts, in both conventional and CASTKR, it is well known to use a cutting guide which assists the surgeonsuch that he or she can make the resection cuts in the right positionand orientation to ensure that the geometry of the knee, oncereconstructed with the prosthetic implants, approaches as much aspossible that of a healthy knee. In many conventional, i.e. non computerassisted, total knee replacement surgery a distal cutting block ispositioned and aligned by the surgeon and pinned in place (typically onthe anterior surface of the femur) such that the cutting slot is alignedin the correct location for the distal cut. In many CAS total kneereplacement surgeries, it is also known to use a distal pin drill guideto accurately create the pin holes into which locating pins are insertedand employed to fix the distal cutting guide, either integrally formedwith the distal pin drill guide or being a separate element, in thecorrect location to make the distal cut in the femoral condyles.Generally, the distal drill/cutting guide member comprises part of anassembly including an anterior guiding platform, that is fixed relativeto the femur and on which the drill/cutting guide is displaceable by aselected, measurable amount to locate the drill/cutting guide in apredetermined position relative to the anterior guiding platform andtherefore relative to the distal end of the femur. A tracked guide blockis often intramedullarly fastened to the femur, and the anterior guidingplatform can then be engaged thereto. Depending on the type of implantbeing used, and once aligned with the most distal femoral condyle, thedrill/cutting guide can then be proximally displaced on the fixedanterior guiding platform by a selected amount corresponding to theamount of bone to be resected.

However, as TKR CAS techniques have developed, a large guide block whichactually abuts the distal end of the femoral condyles is now lesscommonly used, and therefore alternate means are required to determinethe exact location of the most distal point(s) on the femoral condyles,such that this can be used as a reference point from which thepredetermined resection distance can be measured, calculated orotherwise determine, either by the surgeon and/or by the CAS system.Therefore, it is important to the success of the TKR procedure to beable to accurately determine the exact location of the most distal pointon the femoral condyles, such that the distal resection distance can beaccurately measured from this point.

The distal surfaces of the femoral condyles have been more recentlylocated by CAS systems by using a digitizing pointer, which iscalibrated and tracked in space by the CAS system and which is used todigitize a plurality of points on the surfaces of the condyles. The CASsystem then uses these points, which are assumed to be representative ofthe distal condyle surfaces, to plot the condyle surfaces and thusdetermine the location of the most distal point thereof.

Several disadvantages exist with this method of determining the distalsurface in general, and the most distal point thereon in particular. Forexample, such digitizing pointers typically have a relatively sharplypointed tip which can unintentionally penetrate a soft or damaged bonesurface or any soft tissue or cartilage which may be present on the bonesurface, depending on the amount of pressure exerted by the surgeon.While such measurement inaccuracies can be very small (such as 1 mm orless for example), these can nonetheless lead to significant differencesbetween the actual distal surface of the condyles and that determined bythe CAS system, resulting in improperly located distal resection cutsand thus proper fit of the implant. Additionally, while several pointsare taken on the distal condyle surfaces in order for the CAS system todigitize these surfaces, if the points chosen are not representative ofthe entire surface of if they are localized in one portion of the bone,errors in the resultant digitized surface can result. Determining by eyewhich portions of the condyle may be the most distal points, such thatthey can be digitized using the pointer, can also be particularlydifficult on severely deformed or damaged bones.

Therefore, there is a need to provide a CAS system and device whichenables a more accurate and simplified measurement of the most distalpoint(s) on the condyles of a femur prior to knee surgery.

SUMMARY OF THE INVENTION

It is accordingly an object of the present invention to provide animproved CAS system, and method using such a system, capable ofdetermining more accurately the location of a most remote referencepoint, such as a most-distal point for example, on a femoral condyleduring orthopedic knee surgery.

Therefore, in accordance with the present invention, there is provided amethod of determining a reference point on condyles of a femur using acomputer assisted surgery system, the method comprising: providing adigitizer having a trackable member which is locatable and trackable inthree dimensional space by the computer assisted surgery system, thedigitizer including perpendicular first and second planar surfaces;creating a digitized plane using the digitizer, the digitized planebeing at least one of a posterior plane and a distal plane of thecondyles, the posterior plane and the distal plane being respectivelydefined by said first planar surface and said second planar surface ofthe digitizer; and determining a location of the reference point in thedigitized plane using the computer assisted surgical system, thereference point being a most remote point of the femur within saiddigitized plane.

There is also provided, in accordance with the present invention, acomputer assisted surgery system for conducting orthopedic surgery on aknee joint, the knee joint including a femur defining a longitudinalmechanical axis and having condyles on the distal end thereof, thesystem comprising: a condyle digitizer including a posterior condyleabutment surface defining a first plane and a distal condyle abutmentsurface defining a second plane perpendicular to said first plane, and atrackable member which is locatable and trackable in three dimensionalspace by the computer assisted surgery system, thereby defining positionand orientation of the condyle digitizer in real time; and a computerwhich determines a most remote reference point of the femur lying withinat least one of said first and second planes when said condyle digitizeris abutted against the condyles of the femur such that the posteriorcondyle abutment surface abuts a posterior side of the condyles and thedistal condyle abutment surface abuts a distal side of the condyles.

There is further provided, in accordance with the present invention, acomputer assisted surgery system for orthopedic knee surgery comprising:a digitizer having first and second planar surfaces respectivelyabuttable against posterior and distal surfaces of condyles of a femur;means for determining the position and orientation of said first andsecond planar surfaces relative to the femur; and means for determiningthe location of a reference point in a digitized plane defined at leastone of said first and second planar surfaces when abutted against therespective proximal and distal surfaces of the condyles, said referencepoint being a most remote point of the femur within said digitizedplane.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will becomeapparent from the following detailed description, taken in combinationwith the appended drawings, in which:

FIG. 1 is a perspective view of a condyle digitizer and ComputerAssisted Surgery (CAS) system according to the present invention;

FIG. 2 a is a distal end view of a femur;

FIG. 2 b is a lateral side view of the femur of FIG. 2 a;

FIG. 3 is a lateral side view of the femur of FIG. 2 a the condyledigitizer of FIG. 1 abutted thereagainst;

FIG. 4 is an anterior view of FIG. 3;

FIG. 5 is a lateral side view of a femur, such as that of FIG. 2 a; and

FIG. 6 is an anterior view of the femur of FIG. 2 a, showing anapproximation of possible calculation error when using the method of thepresent invention to calculate the location of a distal point on thefemoral condyle.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, the computer assisted surgery (CAS) system 10 isused for orthopedic surgery, and in the present embodiment the system isparticularly employed for conducting total knee replacement (TKR)surgery. The CAS system 10 is capable of real time location and trackingof a plurality of trackable members in a surgical field, each having adistinctive set of identifiable markers thereon. These trackable membersare thus affixed both to the surgical tools employed with the system 10and to the bone elements undergoing surgery.

The CAS system 10 generally includes at least a computer 12, a displaydevice 14 in communication with the computer, and a tracking system 16also in communication with the computer. The tracking system 16 ispreferably an optical tracking system, using infra-red cameras 18,however any other type of tracking system can also be used, such as onebased on laser, ultrasound, electromagnetic or RF waves to locate theposition of the identifiable markers of tracking members within range ofthe sensing devices of the system.

The CAS system 10 is capable of depicting the digitizer 20, as well asany other surgical tools, on the display device 14 (which may include amonitor for example) relative to the patient anatomy, including thepertinent bones which are also tracked in real time by the system. TheCAS system 10 is preferably a CT-less system.

The CAS system 10 includes a digitizer tool 20 which is used inconjunction therewith, the digitizer 20 including a tracking member 22having retro-reflective identifiable markers 19 thereon such that thetracking member 22 is locatable and trackable by the tracking system 16of the CAS system 10, permitting the CAS system 10 such to determine theposition, orientation and movement of the tracking member 22 (andtherefore also the digitizer 20 to which it is fastened) in threedimensional space and in real time. The retro-reflective identifiablemarkers 19 of the trackable member 22 can be removably engaged to eitherside or both sides of the tracking member 22 of the digitizer 20, suchthat the best visual contact between the detectable markers 19 and thecameras 18 of the CAS system 10 is ensured. Therefore, regardless ofwhich knee is being operated on the tracking member 22 will be readilyvisible and detected by the tracking system.

The digitizer 20 includes generally a first end 24 to which the trackingmember 22 is engaged and an opposed second end 26. Between the first andsecond ends 24 and 26 is a handle portion 28 which is adapted to begrasped by the user in order to be able to manipulate the digitizer 20.The second end 26 of the digitizer includes a first condyle abutmentsurface 30 and a second condyle abutment surface 32. In the preferredembodiment, both condyle abutment surfaces are planar andperpendicularly arranged relative to each other. Thus, the first condyleabutment surface 30 defines a first plane and the second condyleabutment surface 32 defines a second plane which is substantiallyperpendicular to the first plane. In the embodiment depicted (see FIG.3) and described in further detail herein, the first condyle abutmentsurface 30 is used to abut the posterior surfaces of the femoralcondyles and the second condyle abutment surface 32 is used to abut thedistal surfaces of the femoral condyles, and thus the first and secondcondyle abutment surfaces are said to be a posterior condyle abutmentsurface and a distal condyle abutment surface respectively.

During CT-less knee surgery, only certain portions of the femur can bedigitized in order to determine essential information. For example, thelongitudinal mechanical axis of the femur can be determined, either withor without digitization. In one embodiment, only the most distal portionof the femur's articular surfaces are digitized. The most distal portionof the femur's articular surface (i.e. on the condyles) is particularlyimportant when it is being used to reference specific bone cuts.Alternately, however, the femur can be fully “constructed” by CAS system10 such that the bone model is as accurate as possible to the anatomy ofthe patient's bone. While several techniques exist to do this, this canbe done using surface model reconstruction, which allows the user todigitize small surfaces instead of points only. When each digitizedsurface has been registered, an approximate model is displayed on anoutput device of the CAS system, such as the display device 14. Usingthis method, the computer femur model is thus formed as a mosaic ofdigitized surfaces. This reconstruction is done in real time,intra-operatively. Regardless of which of these techniques is employed,a digitizing pointer, or registration tool, is commonly used to acquireseveral points and/or larger surfaces on the femur, such that thesesurfaces digitized to form the bone model, which is therefore registered(i.e. the location of the surfaces on the actual bone are matched to thebone model created in the CAS system). During this process, thelongitudinal mechanical axis of the femur is also preferably determined.The longitudinal mechanical axis of the femur generally begins at thecenter of the femoral head (i.e. at the proximal end of the femur),extends the length of the femur, and passes through the center of theinter-condylar notch of the femur. The present invention relatesparticularly to the digitization of the condyles of the femur, and moreparticularly with determining a point on the condyles which is the mostdistally disposed of the femur.

Referring to FIGS. 2 a and 2 b, a partial portion of the distal end of afemur bone 11 is shown, including a medial condyle 13, a lateral condyle15 and an inter-condylar notch 17 therebetween. The pre-determinedlongitudinally extending mechanical axis 21 is also depicted. As the twocondyles 13,15 are typically not the same size or shape, even in healthyknees, one of the two condyles (often, but not necessarily, the medialcondyle 13) will have a point 23 thereon which is the most distal pointon the femoral condyles relative. The present invention permits thedetermination of this most distal point 23. This point is important, asthe CAS system can then measure s from this point to a desired cuttingplane 42 (see FIG. 4), in a direction substantially perpendicularly tothe cutting plane. Similarly, one of two condyles (often, but notnecessarily, the lateral condyle 15) will have a point 25 thereon whichis the most posterior of the femoral condyles. The digitizer 20 and CASsystem 10 permit the coordinates of at least the most distal of the twopoints 23 to be determined in a rapid, accurate and user friendly way,as will be described in further detail below. The most posterior of thepoints 25 can also be similarly determined. In TKR surgery, it isnecessary to be able to determine the distal-most point 23 on thecondyles 13,15 of the femur 11, such as to accurately calculate thenecessary resection distance between this point and the plane throughwhich the resection cut will be made. This is best seen in FIG. 4,described below.

Referring to FIG. 3 and FIG. 4, the condyle digitizer 20 is shown inuse, abutted against the condyles 13,15 of the femur 11. Morespecifically, the posterior condyle abutment surface 30 of the digitizer20 is abutted against the most posterior surfaces of the condyles, andthe distal condyle abutment surface 32 is abutted against the distalsurfaces of the two condyles. As noted above, the planar abutmentsurface 30 of the digitizer 20 defines a posterior plane 29 within whichthe most posterior point 25 of the condyles lies, and the planarabutment surface 32 of the digitizer 20 defines a distal plane 27 withinwhich the most distal point 23 of the condyles lies. As the position andorientation of each of these two planes 27,29 is known, due to theirfixed position relative to the tracking member 22 fixed in place on thedigitizer 20, once the two abutment surfaces 30 and 32 of the digitizer20 have been abutted against their respective condyle surfaces, the CASsystem 10 is thus able to identify the distal plane 27 and the posteriorplane 29 of the condyles 13,15 of the femur 11.

When the surfaces 32 and 30 of the digitizing tool 20 are abutteddirectly against the corresponding surfaces of the condyles, therebydefining the digitized distal and posterior planes 27 and 29respectively, the CAS system 10 then uses a calculation moduleprogrammed therein to determine where on each of these two planes ispositioned the most-distal point 23 and/or the most posterior point 25,respectively. I.e., the CAS system 10 is then able, in the mannerdescribed below, to determine both the Medial-lateral (ML) coordinateand the Anterior-Posterior (AP) coordinate of one or both of thesepoints. Although in FIG. 3 the digitizer is depicted as being positionedsuch that the surface 32 thereof (and therefore the plane 27 also) isoriented substantially vertically, the surgeon may alternately orientthe digitizer 20 as desired or required, for example such that thesurface 32 is positioned at an angle corresponding to the flexion angleof the knee joint or such that the plane 27 of the surface 32 ispositioned substantially parallel to a desired resection cutting plane42, as shown in FIG. 4. In the case that the surface 32 is positioned atan angle corresponding to the flexion angle of the joint, and withreference to the drawing in FIG. 3 showing a lateral side view, thiswould mean that the surface 32 is inclined clockwise. In this case, theAP coordinate of the most distal point 23 need not be determined by themethod described below, as it is then already known and/or determined bythe system because the plane 27 is orientated at the flexion angle andtherefore the contact point of the surface 32 with the condyles willoccur, when disposed in this position, at the most distal location inthe AP direction. As such, only the ML direction coordinates of thispoint need then be determined by the system.

Although this process is described herein with reference to thedistal-most point 23 of the femur 11, one skilled in the art willappreciate that a similar method and technique can be used to similarlydetermine the most posterior point 25 if necessary.

Alternately, the digitizer 20 is provided with a laser marked line on atleast the surface 32, which the surgeon can use to line up with aselected most distal point of either condyle in order to digitize thismost distal point. As the exact position of this laser marked referenceline on the surface 32 is known relative to the tracker 22 of thedigitizer 20, the system can thus determine the location of thedistal-most point on the condyles when aligned in a medial-lateraldirection with this line and abutted against the surface 32. Therefore,while the surgeon positions the digitizer 20 (with the surfaces 30 and32 abutted against their respective condyle faces) such that the distalplane surface 32 thereof is disposed at the desired angle (such asaligned approximately at the predetermined flexion angle of the kneejoint or substantially parallel to the resection cutting plane), he orshe then visually aligns in a medial-lateral direction the laser markedline on the surface 32 with a contact zone of the distal condyle surfaceagainst said surface 32. The CAS system 10 can then measure the distancebetween the resection cutting plane 42 and the digitalized line on thesurface 32 (the line and the cutting plane 42 being approximatelyparallel to each other), thus permitting the determination by the CASsystem of the ML coordinate of the distal point 23 of the femur.Therefore, when such a reference line is provided on the surface 32(permitting the system to determine the ML coordinate of the distalpoint 23), and the above-described method is employed with respect topositioning the surface 32 when acquiring points on the condyles usingthe digitizer 20 (thus permitting the CAS system to determine the APcoordinate of the distal point 23), the CAS system is able to determinethe complete coordinates of the location of the distal-most point 23 onthe condyles of the patient's femur.

When the surgeon is positioning the surface 32 in abutment against thecondyles, this may be done either with both condyles abutting thesurface 32, or alternately with a single condyle (preferably, thevisually most distally extending one) abutting the surface 32, while thesurgeon orients the digitizer 20 such that the plane 27 of the surface32 is substantially parallel to a desired resection cutting plane 42(see FIG. 4).

As discussed in the background section above, when a traditional pointeris used by the surgeon to acquire points on the condyle surfaces such asto digitize their surfaces, identifying the most-distal point on the twocondyles can be error-prone. Some reasons for this include the fact thatthe sharp tip of the digitizing pointer can unintentionally piercecartilage and other bone and/or tissue on the condyle surfaces,resulting in calculation errors. Additionally, particularly when using atraditional digitizing pointer, for certain bone structures it can bedifficult for the surgeon to visually identify exactly which region isin fact the most distal point, such that digitized points can beacquired therearound. In contrast, when using the flat planar surfacesof the digitizer 20 of the present invention, it is much easier for thesurgeon to visually appreciate the most distal point on the femoralcondyle.

The CAS system 10 of the present invention, in an alternate embodiment,also includes an estimation module which, given a general size envelopeof the femoral condyles, can determine with relatively good accuracywhere on the known distal plane 27 the most-distal point 23 is located.This estimation module includes a database, which is pre-stored in thememory of CAS system's computer 12, containing data obtained from asample study of patients, said data including the exact experimentallymeasured coordinates of femur condyles, including the ML and APdimensions, and the measured location of the most distal point thereon.While general differences will occur between patients having differentsized knees, average values given a good population size are believed tobe reasonably reliable. A most commonly employed implant size of a givenmanufacturer in a specific population can be used as a reference frame,for example. Therefore, using averaged values from the database for thecoordinates of the most distal point on the condyles, in conjunctionwith the known distal plane 27, permits the CAS system 10 to determinean expected location for the most distal point 23 on the femur 11undergoing surgery.

If the laser marking on the tool's distal surface 32 is used, asdescribed above, then the CAS system need not use this particularestimation module and the pre-stored database to determine an estimatedmost-distal point. When the laser marking method is used, an accurateline identifying the most-distal point is already known. Using this lineand its known distance from the surface 32, three points can be definedon this laser-marked line which are then fed into the CAS system'scomputing algorithm to determine the location in space, and on thefemur, of the most-distal point on the femur from which cut measurementscan be made.

Alternately, the database values for the location of the distal pointcan be classified into datasets based roughly on the knee size (using agiven implant sizing number as a group classifier, for example). Forexample, once the surgeon has determined the appropriate femur implantsize, this data is intra-operatively inputted into the CAS system 10,either with or without prompting thereby. The CAS system then uses thisinfo to select the corresponding dataset within the database of measuredfemur coordinates. This data includes both the Medial-Lateral (ML)coordinate of the most distal point on each of the condyles measured forthe database and the Anterior-Posterior (AP) coordinate of each samedistal point.

The CAS system 10 will then use the calculated average values for thecoordinates of the most distal point on similar condyles, and then willmatch this information with that of the known distal plane 27 to itselfdetermine a calculated distal point 23 thereon which most likelyrepresents the true distal point of the femur 11. As best seen in FIG.4, from this most-distal point 23, the CAS system 10 can then calculatea necessary resection distance 40 proximally inward therefrom. Thisresection distance 40 is thus defined based on the determined locationof the resection cut plane 42 through which the femur will be cut by thesurgeon to resect the predetermined amount required to install thechosen implant in place on the end of the femur. This resection distance40 will correspond to the type and size of implant being used. Althoughthe resection cut plane 42 is preferably approximately perpendicular tothe longitudinal mechanical axis 21 of the femur as shown in FIG. 4, andtherefore not parallel to the distal plane 27, it can alternately beparallel to the distal plane 27. Ideally, the distal resection cut ismade substantially perpendicular to the mechanical axis. The minimumresection level is chosen by the surgeon such that the damaged portionof the bone is removed, without removing more than is judged necessary,and such that the chosen implant can be installed in place on the femur.

Several other methods can also be used to determine and provide the CASsystem 10 with the general full-scale dimensions (i.e. the fullMedial-Lateral (ML) measurement and the full Anterior-Posterior (AP)measurement—see FIGS. 2 a-2 b) of the femoral condyles 13,15, from whichthe CAS system 10 is capable of determining, as described above, anapproximate position of the most distal point 23 of the femur within theknown distal plane 27. The overall size envelope (i.e. the total ML andAP measurements) of the femoral condyles 13,15 of the femur 11 can alsobe determined, rather than using the averaged values of the measuredpopulation used to build the database described above, using either one(or both) of the following alternate methods.

As shown in FIG. 5, the total AP and ML measurements of the femur 11 canbe determined by the CAS system's software, by manually digitizingseveral individual points 38 on both the anterior and posterior surfacesof the femur's condyles 13,15, such that the CAS system 10 can thencalculate, based on these points 38, the AP distance of the femur 11.The CAS system 10 can then use this measured dimension of the femur 11to search the database for similarly sized femurs, and employ the ML andAP coordinates of the most distal points of similarly sized femurs fromthe database to associate with the known distal plane 27, such that acalculated distal point 23 on this plane 27 is determined by the systemto approximate the true most-distal point on the measured femur. Ratherthan measuring the AP distance of the femur 11 intra-operatively bydigitizing points 38 thereon, in another alternate methodpre-operatively taken images (such as an x-ray for example) areevaluated to associate a template size, such as one corresponding toimplant size for example, which can then be imputed in the CAS system,and then determines the approximate most distal point 23 using thedatabase values as described above. This evaluation may include actualmeasurement of the x-ray images, or mere qualitative evaluation by thesurgeon to determine appropriate implant size, and thus subset ofdatabase values used to determine the distal-most point 23 on the distalplane 27. While these methods are proposed and thus preferably used,either alone or in combination, to determine these dimensions, oneskilled in the art will appreciate that other means of determining thefull scale ML and AP dimensions of the patient's femoral condyles are ofcourse also possible.

While the determination by the CAS system 10 of the location of the mostdistal point 23 in the distal plane 27 is an approximation, calculatedusing averaged literate or measured values, the possible error in thiscalculation (i.e. between a true most distal point on a femur's condylesand the theoretic or calculated one determined by the CAS system asdescribed above) is relatively small. In fact, this error is believed tobe much less than current errors in measurement which occur due to theproblems described above with respect to the prior art techniques. Forexample, referring to FIG. 6, a case in which the predeterminedresection distance 40 required is 10 mm and the femur's naturalvarus-valgus angle θ is 10 degrees is depicted, for the purposes ofcalculating a worst case resection error value when the above-describedmethod is used by the CAS system 10 to approximate the locations of themost-distal point 23 on the femur. The point 23 on the known distalplane represents the location of the point calculated by the system tobe the most distal point on the condyles of the femur in question. Thepoint 46 represents the true most-distal point. Given that the size ofmost femoral condyles, while varying from person to person, does notvary by huge amounts, assuming for the present calculation an errordistance 52 (i.e. between the true distal point 46 and the distal point23 calculated by the CAS system 10) in the ML direction to be 5 mm. ByPythagoras's theorem, the resection distance error 50 is equal to the MLerror distance 52 of 5 mm multiplied by the sine of the varus-valgusangle θ of 10 degrees. This gives a possible resection error distance 50of less than 1 mm, which given this is a worst case scenario, isbelieved to be very acceptable.

Although the present invention is preferably used with a CT-free CASsystem 10, it is nevertheless to be understood that the step ofperforming a digitization or registration of the bone element, asdescribed above, comprises all means of relating the actual bone elementto a corresponding model or image of the same bone element. Thoseskilled in the art will appreciate that there are a plurality of ways ofcreating such a model or image of the bone element, and of relating ormatching the actual bone element to the model or image thereof. Allsimilarly suitable methods of generating a computerized model ordisplaying image of the bone element, and of relating or matching theposition and orientation of the actual bone element thereto, will beunderstood herein to be included in the process of performing aregistration of the bone element.

All surfaces of the digitizer 20 can be easily cleaned. Particularly,all surfaces can be sufficiently exposed such that thorough pressurecleaning is possible. The ability to sterilize all surfaces of thedevice by pressure cleaning and autoclaving is important to ensure thatall potentially contaminating biological matter can be safely removed.Such potentially dangerous contaminating biological matter can includeunwanted bacteria and proteins, which can cause infections or diseases.

Although throughout this specification the present invention isgenerally described with regard to its use in total knee replacement(TKR) surgery, it is to be understood that the present invention can besimilarly used in conjunction with all types of orthopedic knee surgery,for example partial knee replacement, knee repair operations, and thelike. Further, although the present invention is described hereinprincipally with regard to determining a most-distal point on the femur,it is to be understood that the same method and system described canalso be used to determine a most proximal end point on a tibia. Furtherstill, although the two perpendicular abutting surfaces of the condyledigitizer described herein are used in the embodiment described anddepicted to determine points on the distal surface and/or the posteriorsurface of the femoral condyles, one skilled in the art will appreciatethat by inverting the tool relative to the femur the same condyledigitizer of the present invention can be used, if necessary, todetermine an anterior point on the anterior surface of the condyles.

The embodiments of the invention described above are intended to beexemplary only. The scope of the invention is therefore intended to belimited solely by the scope of the appended claims.

1. A method of determining a reference point on condyles of a femurusing a computer assisted surgery system, the method comprising:providing a digitizer having a trackable member which is locatable andtrackable in three dimensional space by the computer assisted surgerysystem, the digitizer including perpendicular first and second planarsurfaces; creating a digitized plane using the digitizer, the digitizedplane being at least one of a posterior plane and a distal plane of thecondyles, the posterior plane and the distal plane being respectivelydefined by said first planar surface and said second planar surface ofthe digitizer; and determining a location of the reference point in thedigitized plane using the computer assisted surgical system, thereference point being a most remote point of the femur within saiddigitized plane.
 2. The method as defined in claim 1, wherein the stepof determining includes consulting a database of coordinates of measuredpoints on femoral condyles, using an average value thereof to calculatea theoretical most remote point on said distal plane.
 3. The method asdefined in claim 2, further comprising consulting a subset of saiddatabase containing said measured points of condyles having a similarsize variable to said femur.
 4. The method as defined in claim 3,further comprising determining the size variable of said femur.
 5. Themethod as defined in claim 4, wherein said size variable is a totaldimension of said femur, said total dimension being determined bydigitizing points on said femur.
 6. The method as defined in claim 1,wherein the reference point is a distal-most point of the femur, thestep of determining includes identifying the distal-most point withinsaid distal plane.
 7. The method as defined in claim 6, furthercomprising using the computer assisted surgery system to determine alocation of a distal resection cut, including calculating a resectiondistance away from said distal-most point.
 8. The method as defined inclaim 7, further comprising displaying at least one of said resectiondistance and said location of the distal resection cut on a displaydevice of the computer assisted surgical system.
 9. The method asdefined in claim 1, further comprising using the computer assistedsurgery system to determine and display position and orientation of saiddigitizer relative to the femur.
 10. The method as defined in claim 1,wherein the step of creating the digitized plane includes abutting thefirst planar surface of the digitizer against posterior surfaces of thecondyles and abutting the second planar surface against distal surfacesof the condyles.
 11. The method as defined in claim 10, furthercomprising orienting the second planar surface of the digitizer at apredetermined angle.
 12. The method as defined in claim 11, wherein thepredetermined angle includes one of a first angle substantially parallelto that of a desired resection cut and a second angle substantiallyparallel to a flexion angle of a knee joint of said femur.
 13. Themethod as defined in claim 1, wherein the step of determining thelocation of the reference point in the digitized plane furthercomprises: providing a database of coordinates of most remote points onpre-measured femoral condyles; determining a size characteristic of saidfemur; using said size characteristic to select a subset of data valuesfrom said database corresponding to similarly sized femurs; determiningan average value of said coordinates from said subset; and determiningthe location on said femur of a theoretical most remote point thereonbased on said average value and said digitized plane.
 14. The method asdefined in claim 1, wherein the step of determining includes determiningan Anterior-Posterior coordinate and a Medial-Lateral coordinate of thereference point in the digitized plane.
 15. The method as defined inclaim 1, wherein the step of determining includes aligning a lasermarked line, provided on at least on of the first and second planarsurfaces of the digitizer, with a visually selected most-remote point onthe condyles.
 16. The method as defined in claim 15, further comprisingusing the computer assisted surgery system to determine a distancebetween a resection plane of a desired resection cut and the lasermarked line on the digitizer.
 17. A computer assisted surgery system forconducting orthopedic surgery on a knee joint, the knee joint includinga femur defining a longitudinal mechanical axis and having condyles onthe distal end thereof, the system comprising: a condyle digitizerincluding a posterior condyle abutment surface defining a first planeand a distal condyle abutment surface defining a second planeperpendicular to said first plane, and a trackable member which islocatable and trackable in three dimensional space by the computerassisted surgery system, thereby defining position and orientation ofthe condyle digitizer in real time; and a computer which determines amost remote reference point of the femur lying within at least one ofsaid first and second planes when said condyle digitizer is abuttedagainst the condyles of the femur such that the posterior condyleabutment surface abuts a posterior side of the condyles and the distalcondyle abutment surface abuts a distal side of the condyles.
 18. Thecomputer assisted surgery system as defined in claim 17, furthercomprising a display device which graphically indicates a location ofthe determined most remote reference point on the femur.
 19. A computerassisted surgery system for orthopedic knee surgery comprising: adigitizer having first and second planar surfaces respectively abuttableagainst posterior and distal surfaces of condyles of a femur; means fordetermining the position and orientation of said first and second planarsurfaces relative to the femur; and means for determining the locationof a reference point in a digitized plane defined at least one of saidfirst and second planar surfaces when abutted against the respectiveproximal and distal surfaces of the condyles, said reference point beinga most remote point of the femur within said digitized plane.
 20. Thesystem as defined in claim 19, further comprising means for determiningand indicating a location of a distal resection cut calculated by thecomputer assisted surgery system based on a resection distance measuredfrom said reference point.